1. Field of the Invention
This invention relates to angular substrates for use in the semiconductor technology.
2. Prior Art
As the demand for higher DRAM integration level and smaller microcircuit geometry has continued to increase, so has the demand for chips of larger size. Accordingly, photomasks have a larger exposure region and are thus required to have a higher degree of flatness even to their periphery.
In one example wherein a square substrate is 152 mm×152 mm (6 inches) and a chip has a size of 30 mm, the exposure region of a photomask is 120 mm in the case of 4-fold reduction exposure, and reaches 150 mm in the case of 5-fold reduction exposure. In fact, it rarely happens that the substrate is utilized to the extremity of 150 mm as the exposure region. However, since alignment marks or the like are necessary outside the exposure region, a high degree of flatness is required even to the periphery of the substrate.
In another situation, a substrate is rested on a sample holder of an inspection instrument. For example, a substrate 1 is mounted on a sample holder in the form of spaced blocks 2 as shown in FIG. 8, or a substrate 1 is mounted on a sample holder in the form of pins 3 as shown in FIG. 9. In either case, the sample holder supports the substrate at its periphery. Since the exposure region is enlarged and alignment marks or the like are provided outside as mentioned above, the portion of the substrate which is available for attachment by the sample holder is shifted more outward.
As a result, not only the exposure region and the alignment mark-bearing region of a substrate, but also a peripheral region thereof is now required to have a certain degree of flatness. If the peripheral region of a substrate has a low degree of flatness, there would arise a problem that when the substrate is mounted on a sample holder, the substrate is not kept horizontal, causing a lowering of inspection sensitivity and a failure of reproduction.
Another problem arises when a resist solution is spin coated onto a substrate. For example, as shown in FIG. 10, a sample holder 6 is provided along the periphery of a stage 7. While the stage 7 with a substrate 1 mounted on its sample holder 6 is rotated, a resist solution is applied to the substrate 1 at the center. To prevent the substrate 1 from moving aside during the spin coating, the substrate 1 is secured to the stage 7 by vacuum chucking through a suction hole 8 in the stage 7. The sample holder 6 is often made of a synthetic resin such as polyacetal resin. In this case, the sample holder supports the substrate along its periphery as well. If the substrate has a low degree of flatness at its periphery, the predetermined vacuum would not be reached upon vacuum suction and the precision of a resist coating surface would be adversely affected.
Referring to FIG. 1, a method for polishing an angular substrate using a general single-side polishing machine is described. A polishing turntable 13 includes a platen 11 and a polishing pad or cloth 12 attached thereto. An abrasive fluid 14 is fed to the polishing pad 12 at its center via an abrasive supply line 15. A substrate holding head 16 holds a substrate to be polished (not shown) and presses it against the polishing turntable 13. By independently rotating the turntable 13 and the substrate holding head 16 in this state, the substrate is polished.
FIG. 2 is a schematic view showing only the turntable 13 and the substrate to be polished 1. The substrate 1 is abraded while it rotates about its center. The substrate surface is then polished in a concentric pattern.
However, since corner areas (hatched areas in FIG. 2) 1a of the substrate 1 out of the inscribed circle are readily subject to the rapid removal of material, the corner areas 1a of the substrate 1 tend to undergo excessive polishing. This is due in part to differences across the substrate 1 in its relative velocity with the polishing turntable 13. The main reason why the corner areas 1a of the substrate 1 tend to undergo excessive polishing is explained by referring to FIG. 3 which is a schematic view showing only the polishing pad 12 and the substrate to be polished 1. When the substrate 1 is pressed against the polishing pad 12 during polishing, it sinks into the polishing pad 12 under the pressing force in the manner shown in FIG. 3. At the start of such sinking, the polishing pad 12 exerts an elastic force upon the substrate 1 to increase friction by the polishing pad 12, facilitating material removal in the corner areas 1a. In addition, because the substrate 1 is an angular substrate, the polishing pad 12 which polishes the corner areas 1a alternately undergoes the application and release of pressing forces from the substrate 1. Inversely, the substrate corner areas 1a have repeated chances of incurring the elastic forces of the polishing pad 12. As a result, the corner areas 1a are over-polished, leaving difficulties in achieving flatness across the corner areas 1a. In FIG. 3, the lengths of the arrows indicate the relative magnitude of the forces of restitution acting under the elasticity of the polishing pad 2.